The symptom is that, upon startup, bridged network guest (in my case, both Ubuntu 12.04 and 13.04) throw an error (which I forgot exactly what now, but something to do with the network adapter). It’ll run just fine in NAT mode.

I got both of these errors when I upgraded to VirtualBox 4.1.2! Yikes! Luckily, both were solved for me by fixing the permissions on the /Applications directory. The VirtualBox forums have the details, links below.